Method for detecting slippery surfaces and blocked vehicle wheels, automatic transmission implementing the method and vehicle equipped with same

ABSTRACT

A method for detecting slippery ground in a vehicle with automatic transmission includes detecting the instantaneous running conditions, with the aid of at least one parameter that represents the existence of slippery ground; on the basis of this detection, generating, by counting, a value that represents the change in running conditions as a function of time; comparing the value with reference conditions; as a function of the result of the said comparison, deciding whether or not the instantaneous running conditions correspond to slippery ground.

BACKGROUND OF THE INVENTION

The invention relates to automatic transmissions for land vehiclesequipped with gearboxes with discrete gear ratios or constantly varyingtransmission ratios. It relates more specifically to a method fordetecting slippery ground on which the driving wheels of a vehicle havea tendency either to spin or to lock up.

DESCRIPTION OF THE INVENTION

A number of methods of controlling automatic transmissions, having agreater or lesser ability to take particular running conditions intoconsideration, are already known in the state of the art.

Thus, document FR 83/07277 describes a control method intended toeliminate inopportune up-shifting from a ratio N−1 to a higher ratio Nin a phase when the driver has lifted off. However, this document doesnot tackle the problems associated with the spinning or locking of thedriving wheels.

Another prior document, EP-A-0,638,742, describes the control of anautomatic transmission using variables associated with the load of thevehicle, with the driver's more or less sporty driving style, and withthe advisability of a change in gear ratio. These variables are obtainedusing fuzzy logic, as a function of a number of input parametersincluding conditions of the spinning of the driving wheels. However, theway in which the wheelspin or wheel lock-up conditions are taken intoaccount, and the detection of the existence of slippery ground, are notdescribed.

Furthermore, in most of the known methods for managing gear changes(continuous or discrete) in automatic mode, the driver has the option ofmanually selecting, using a switch, a particular driving mode tailoredto slippery ground, and generally known as the “winter” or “snow” mode.

SUMMARY OF THE INVENTION

An object of the invention is to assist the driver in the particularsituations mentioned previously, so as to guarantee optimum safety andthe best possible driving comfort.

To this end, the invention proposes a method comprising steps consistingin:

detecting the instantaneous running conditions, with the aid of at leastone parameter that represents the existence or otherwise of slipperyground;

on the basis of this detection, generating, by counting, a value thatrepresents the change in running conditions as a function of time;

comparing the said value with reference conditions;

as a function of the result of the said comparison, deciding whether ornot the instantaneous running conditions correspond to slippery ground,

characterized in that in order to generate, by counting, a value thatrepresents the change in running conditions as a function of time, acurrent value is decremented by a first step (Δx_ΔB) each time lockingof the driving wheels is detected, the said current value is decrementedby a second step (Δx_ΔE) each time the spinning of the driving wheels isdetected, and the said current value is incremented by a third step(Δx_out) when the wheels are not locked or spinning.

According to other features of the method:

the said reference conditions with which the value representing thechange in running conditions is compared consist of a lower threshold(S1) and an upper threshold (S2), and it is decided that the ground isslippery when the said value becomes higher than the lower threshold(S1), and it is decided that the ground is not slippery when the saidvalue becomes lower than the upper threshold (S2).

advantageously, the lower threshold (S1) and the upper threshold (S2)are variable over time and are set as a function of the driver's drivingstyle.

as a preference, the incrementation step (Δx_out) is set as a functionof the driver's driving style.

The invention makes provision, when slippery ground is detected, for atleast one of the following actions to be taken:

the automatic transmission is switched into a safety operating mode,particularly the so-called “snow” mode to provide drive to the vehicle;

an indicator alerting the driver to the fact that the safety mode hasbeen adopted is activated;

the function of down-shifting early under braking is cancelled so as notto down-shift if the wheels are locked.

In order to obtain the intended results, the invention therefore reliesfirstly on a principle of detecting slippery ground, this principlebeing based essentially on analysing the gradient of a parameter thatrepresents the speed of the driving wheels. This analysis makes itpossible automatically to activate a special mode for managing thetransmission ratios, this mode being tailored to low adhesion, and this,on the one hand, provides the driver with greater safety and, on theother hand, affords driving comfort that is optimized with respect tothe specific running conditions.

Automatic switching to “snow” mode allows the driver to obtain optimizedmanagement of the automatic transmission ratios for the runningconditions encountered on slippery ground.

This makes it possible, for example, when pulling way with wheelspin, tobe in a transmission ratio which provides the vehicle with drive andwhich, when the wheels lock up, makes it possible not to engage lowratios at high speed, as this would be ill advised from the safety pointof view.

Furthermore, in order to detect a condition in which the driving wheelsare locked up, the derivative of a characteristic variable of therotation of the driving wheels is analysed under braking, and a lockingof wheels is detected if the brake is active and if the said derivativeis negative and below a threshold (Sb) representative of locking.

Furthermore, to detect the end of a condition of locking, the methodaccording to the invention comprises the following steps:

(C1): testing whether the brake is released and whether the vehiclespeed is below a first threshold (Svv1) and, if these conditions aresatisfied, starting a first time delay (T1);

(C2): testing whether the vehicle speed is above a second threshold(Svv2) or whether the selected transmission ratio is higher than orequal to the current ratio and, if one of these conditions is satisfied,starting a second time delay (T2);

(C3): testing whether one of the said time delays (T1,T2) has elapsed,whether the automatic transmission is in the P, R or N position, orwhether the “kick-down” is activated,

and if one of these conditions (C1,C2,C3) is satisfied, detecting theend of a condition in which the driving wheels are locked.

The invention also relates to an automatic transmission employing theabove control methods and to a motor vehicle with an automatic gear boxwith discrete gear ratios equipped with such an automatic transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages of theinvention will become clear from reading the following description givenby way of non-limiting example and in the appended drawings, in which:

FIG. 1 depicts a block diagram of the steps of the method for detectingslippery road according to the invention, and the input and outputparameters used by the method;

FIG. 2 depicts the principle of recognizing the state of the slipperyroad by managing a recognition counter which is decremented orincremented according to the running circumstances;

FIG. 3 depicts the principle of detecting the state of the slippery roadas a function of the state of the slippery ground recognition countercompared with predetermined thresholds;

FIG. 4 depicts, in the form of a graph of the throttle valve angle as afunction of vehicle speed, a characteristic cycle of the problem ofdown-shifting through the ratios when the wheels are locked, inherent inthe very principle of managing the ratios in automatic transmissions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is made to FIG. 1. This depicts the method for detectingslippery ground in the form of two interconnected functional blocksdepicting the main steps in the detection method. Each functional blockreceives inputs and generates an output which is sent to the next block,the output of the last block representing the desired information,namely whether or not the road is slippery.

A first functional block 1 corresponds to a first step in the method fordetecting slippery ground, which consists in generating a value in acounter, the change and instantaneous state of which quantify the moreor less slippery nature of the ground, as will be explained in furtherdetail later.

To construct the output value at 2 on the slippery road recognitioncounter, use is made, at input 3 to the functional block 1, of an itemof information about the detection of the locking of the driving wheels,which information item is available from elsewhere, and, at input 4 tothe functional block 1, of an item of information about the detection ofthe spinning of the driving wheels of the vehicle, which informationitem is also available from elsewhere and, as is known, for example froma device (ASR) for detecting and regulating wheelspin.

As an alternative, it is possible to use, at 5, in place of the item ofinformation about detection of the locking of the wheels, an “ABSactive” information item (if the vehicle has ABS (anti-lock brakingsystem)), and, on the other hand, at 6, in place of the information itemabout the detection of the spinning of the wheels, an “ASR active”information item (if the vehicle has ASR (anti-spin regulation)).

In a second step of the method for detecting slippery ground, a secondfunctional block 7 determines, as a function of the value at output 2from the slippery ground recognition counter, whether or not the road isslippery, taking account of external parameters (8,9) supplied to thefunctional block 7. To this end, to determine whether or not the road isslippery, the instantaneous value of the counter (at 2) is compared withpredetermined thresholds S1,S2 as will be explained in conjunction withFIG. 3.

When the result of this comparison indicates that the road is recognizedas being slippery, the following actions, separately or in combination,are decided upon in the transmission computer:

a) switching to the so-called “snow” mode for driving comfort andsafety. In this mode, only the higher ratios are available for thetransmission, which therefore transmits a more moderate torque to thewheels, preventing them from spinning.

b) illuminating the “snow” warning light, if available, which makes itpossible to alert the driver to the fact that the road is slippery;

c) cancelling the “early down-shifting under braking” function, ifavailable in the computer, which makes it possible to limit enginebraking and therefore the braking torque applied to the wheels, forsafety reasons, such as for the reason of avoiding locking.

Reference is made to FIG. 2 for a more detailed description of how theoutput state (at 2) on the recognition counter is obtained and how it isused to determine whether or not the road is slippery.

In this figure, time is plotted along the abscissa and the value of theslippery road recognition counter is plotted on the ordinates, as afunction of time. Starting from a given initial value, for example amaximum value of 255, corresponding to the coding over 8 bits of thevalue of the counter, the instantaneous value of the recognition counteris determined as follows:

when the spinning of the driving wheels is detected at 4 or when theanti-spin regulation ASR at 6 (if available in the vehicle) is inoperation, then the slippery road detection counter is decremented by alump-sum step Δx_ΔE, every 320 ms for example;

when the locking of the driving wheels is detected at 3, or when theanti-lock ABS system at 5 (if present on the vehicle) is in operation,the slippery road recognition counter is decremented by a lump-sum stepΔx_ΔB;

when no spinning is detected and the ASR is deactivated, or when nolocking is detected and the ABS is deactivated, then the slippery roadrecognition counter is reincremented with a step denoted Δx_out,signifying that the method is coming out of a “slippery road” decision.

It should be noted that the decrements Δx_ΔE and Δx_ΔB represent a shifttending towards a “slippery road” decision. Furthermore, the fact ofenvisaging decrementation steps Δx_ΔE, Δx_ΔB which are different forwheelspin and for locking, in particular makes it possible to obtaindifferent sensitivity towards detecting a slippery road depending on thesituation provoked by the driver (locking or spinning). The values ofthe Δx_ΔE, Δx_ΔB and Δx_out steps can easily be chosen by the personskilled in the art, taking account of course of the scale of countervalues available, namely 256 values in the example described.

Furthermore, the choice of two different thresholds S1,S2 for the inputand output of the slippery road condition, introduces an hysteresiseffect which makes it possible to avoid any phenomenon of huntingbetween two states of the automatic transmission.

Incidentally, it could very well be envisaged for there to bedecrementation steps which are also a function of the driver's drivingstyle, for example for a driver who more readily than a calmer drivercreates wheelspin and locks his wheels, and for this calmer driver,recognition of a slippery road could be made less sensitive.

Likewise, the incrementation step Δx_out could also depend on thedriver's driving style so as not to remain in “snow” mode too long, ifthe driver is a “sporty” driver. The method for detecting slippery roadaccording to the invention therefore leaves scope for customizedadjustments, possibly made adaptively.

Similarly, if a step downwards slope is detected, the risks of thewheels locking are increased and may, in this case, desensitizerecognition of a slippery road in favour of detecting the locking of thewheels, which then takes precedence.

Reference is made to FIG. 3 which gives the input and output thresholdsin the so-called slippery road state. Thus, when the slippery staterecognition counter gradually drops below a first detection thresholddenoted S1=Sbr_in, (also depicted in broken lines in FIG. 2), then theroad is detected as being slippery, and one of the measures (a,b,c)described above, for example, is taken.

Then, starting out from a low value (below S1) the counter once againbecomes higher than a second detection threshold denoted S2=Sbr_out,then the road is detected as no longer being slippery, and the safetymeasures (a,b,c) taken earlier can be cancelled. The introduction ofsuch a phenomenon of hysteresis between the input at S1 and the outputat S2 of the so-called slippery state makes it possible to avoidphenomena of oscillations relating to the detection and makes itpossible to manage the input and the output independently in thedetection mode in question.

As has been seen, the method for detecting slippery ground according tothe invention calls in particular upon a step of detecting the lockingof the driving wheels. One embodiment of this method for detectinglocking will now be described in conjunction with FIG. 4, it beingunderstood that other methods for detecting the locking of drivingwheels may be envisaged without detracting from the generality of themethod described above for detecting slippery ground.

In most gear change control strategies in automatic mode, the gearratios are selected as a function of the vehicle speed and of the loadon the engine. In most cases, these criteria are translated into theform of gear change laws, conveniently mapped in a plane Vveh/αth, Vvehdenoting the vehicle speed, and ath denoting the throttle angle or, moregenerally, the angle of opening of the member that regulates the inletof fuel to the engine.

Thus, a driver driving along at a certain speed V in gear N will, if hebrakes for example on ground with a low adhesion causing the drivingwheels to lock, cross the down-shift curve 12 of FIG. 4 at the point B.This will cause down-shifting from a first gear ratio on and so forthuntil the wheels are completely locked which corresponds to the point Cwhere 1st gear (in the case of a gearbox with discrete gear ratios) orthe minimum ratio (in the case of a constantly varying transmission) hasbeen engaged. This is very dangerous to the driver since there is thepossibility that one or even more inopportune down-shifts will be madeafter the wheels have locked, particularly at high speed.

The object of the present invention is therefore also to correct thisshortcoming, by avoiding the down-shifts customarily brought about bythe detection of the locking of the wheels, and allowing the driver topull away again in the “right” ratio when the locking of the wheels isno longer detected. In addition to improving safety, this aspect of theinvention appreciably improves the pleasure of driving, because itavoids sharp engine braking and possible loss of control when the wheelslock.

The detection of the locking of the wheels essentially relies onanalysing the derivative of a characteristic variable of the rotation ofthe driving wheels (from a vehicle speed sensor or a torque converterturbine speed sensor) under braking.

By way of example, the principle of detecting the locking of the drivingwheels is based on an analysis of the rotational speed of the drivingwheels. This makes it possible to appropriately manage the gear ratiosin phases when the wheels are locked, and not to down-shift through theratios at inopportune moments, particularly at high speed. Thisguarantees greater driving safety, particularly on slippery ground orground with a low adhesion, as described earlier.

Table 1 which follows represents, in terms of functions, the principleof detecting the locking of the wheels with conditions of input to thedetection function, actions decided upon if locking is detected andconditions for exiting the wheel locked detection.

TABLE 1 FUNCTION OF DETECTING THAT THE WHEELS ARE LOCKED Input functionBrake active and dVveh/dt < locking_threshold < 0 Actions Imposes 3rdgear if in 4th gear, otherwise stays in current gear “current gear”Output C1: {if brake off and Vveh < Svv1} then conditions {start timedelay T1} C2: {if Vveh > Svv2 or if gear_selected ≧ current_gear} thenstart time delay T2 C3: {T1 elapsed} OR {T2 elapsed} OR {lever = P,R orN} OR {“kick-down” activated}

Reference is made to Table 1. According to the invention, when thedriver is braking and the wheels are locking, looking at the derivativeof the vehicle speed makes it possible quickly to detect locking of thedriving wheels, which corresponds to the conditiondVveh/dt<locking_threshold<0. According to the method for detecting thelocking of the wheels, the action in response to detection consists instaying in the current gear, which makes it possible not to down-shiftthrough all the gears (possibly even as far as first gear). At the sametime, the conditions C1,C2,C3 for coming out of this state of staying ingear are constantly tested.

The condition (C1) allows the condition of detecting that the wheels arelocked to be left when the brake is released because in this case therecan be no locking of the wheels. If, for a certain period of time T1,the speed of the vehicle remains below a threshold SVV1 (a minimumcalibratable threshold for example), then this means that the vehicle isactually stationary, and detection that the wheels are locked may cease.The gear corresponding to zero speed will then be engaged.

By contrast, the condition (C2) makes it possible, for example, when thedriver releases the brakes after locking the wheels, to wait until thespeed returns to its “normal” operating point according to the gearchange laws, and this is what is also managed by a time delay T2 (of theorder of 300 to 500 ms in practice). This allows the vehicle to pullaway again in the “right” transmission ratio, which improves thepleasure of driving.

The various steps in the method for detecting slippery ground accordingto the invention may be implemented by an automatic-transmissionelectronic management system which is within the competence of theperson skilled in the art, particularly by adapting existing managementsystems. This system will not therefore be described in itself.

The condition (gear_selected≧current_gear) means that if the ratioproposed by the gear-change laws is higher than or equal to the currentgear ratio, detection of the locking of the wheels may cease. Otherconditions may also be taken into consideration such as, for example, ina C3, the position of the gear lever (P,R or N) or the “kick-down”lever, if the driver wishes to down-shift, which is a safety condition.

According to another feature of the method for detecting locking, inorder to determine the input condition for the wheel locking phase, itis also possible to observe the variations in a variable which signifiesthe rotation of the torque converter turbine in addition to, oralternatively in place of the vehicle speed, provided, however, thelocking threshold is dependent on the current ratio.

In short, the method according to the invention makes it possible todetect slippery ground and react to this detection by imposing on theautomatic transmission operating conditions which are capable ofincreasing both driver safety and driving pleasure.

What is claimed is:
 1. A method of detecting slippery ground for avehicle with an automatic transmission, comprising the steps of:detecting an instantaneous running condition of driving wheels;generating a revised condition value representing a time change in therunning condition of the driving wheels by decrementing a current wheelcondition value by a first step (Δx_ΔB) each time a locking of thedriving wheels is detected in the detecting step, decrementing thecurrent wheel condition value by a second step (Δx_ΔE) each time aspinning of the driving wheels is detected in the detecting step, andincrementing the current wheel condition value by a third step (Δx_out)each time neither a locking of the driving wheels nor a spinning of thedriving wheels is detected in the detecting step; comparing the revisedcondition value with a reference condition value; and based on thecomparison of the revised condition value with the reference conditionvalue, deciding whether or not the instantaneous running conditioncorresponds to a slippery ground condition.
 2. The method of claim 1,wherein, after the deciding step decides that the instantaneous runningcondition corresponds to a slippery ground condition, the methodcomprises the further step of switching the vehicle's automatictransmission into a safety mode.
 3. The method of claim 1, wherein,after the deciding step decides that the instantaneous running conditioncorresponds to a slippery ground condition, the method comprises thefurther step of activating an indicator alerting a driver that a safetymode has been adopted.
 4. The method of claim 1, wherein, after thedeciding step decides that the instantaneous running conditioncorresponds to a slippery ground condition, the method comprises thefurther step of de-activating a capability of early down-shifting duringbraking wherein during braking and the locking of the driving wheelsthere is no early down-shifting.
 5. The method of claim 1, wherein, thestep of detecting the instantaneous running condition of the drivingwheels is performed by detecting a spinning of the driving wheels. 6.The method of claim 1, wherein, the reference condition value comprises:a lower threshold value (S1) and an upper threshold value (S2), andwherein the deciding step determines that the instantaneous runningcondition corresponds to a slippery ground condition when the revisedcondition value becomes greater than the lower threshold value and thatthe instantaneous running condition does not correspond to a slipperyground condition when the revised condition value becomes greater thanthe upper threshold value.
 7. The method of claim 6, wherein, the lowerand upper threshold values are variable over time and are set based on adriving style.
 8. The method of claim 6, wherein, the third step(Δx_out) is set based on a driving style.
 9. The method of claim 1,wherein, the step of detecting the instantaneous running condition ofthe driving wheels is performed by detecting a locking of the drivingwheels.
 10. The method of claim 9, wherein the detecting of the lockingof the driving wheels comprises analyzing, under a braking condition, aderivative of a characteristic variable of a rotation of the drivingwheels by the steps of testing whether a brake is released and whether avehicle speed is below a first speed threshold (Svv1), starting a firsttime delay (T1) when the brake is released and the vehicle speed isbelow the first speed threshold; testing whether the vehicle speed isabove a second threshold (Svv2) and whether a selected transmissionratio is at least equal to a current transmission ratio, starting asecond time delay (T2) when the vehicle speed is above a secondthreshold (Svv2) or the selected transmission ratio is at least equal tothe current transmission ratio; testing whether one of the first andsecond time delays have elapsed, whether the automatic transmission isin one of a P, R, and N position, and whether a kick-down is activated;and detecting an end of a driving wheels locked condition upon thetesting steps determining any of the brake being released and the avehicle speed being below the first speed threshold (Svv1), the vehiclespeed being above the second threshold (Svv2), the selected transmissionratio being at least equal to the current transmission ratio, one of thefirst and second time delays having elapsed, the automatic transmissionbeing in one of the P, R, and N positions, and the kick-down beingactivated, wherein the locking of the driving wheels is detected when,under the braking condition, the derivative is negative and below athreshold locking valve (Sb).
 11. The method of claim 9, wherein thedetecting of the locking of the driving wheels comprises the step ofanalyzing, under a braking condition, a derivative of a characteristicvariable of a rotation of the driving wheels, and wherein the locking ofthe driving wheels is detected when, under the braking condition, thederivative is negative and below a threshold locking valve (Sb).
 12. Themethod of claim 11, wherein the characteristic variable is a vehiclespeed.
 13. The method of claim 11, wherein the characteristic variableis a torque converter turbine speed.
 14. An automatic transmission,comprising: a detector for detecting an instantaneous running conditionof driving wheels; and a processor for i) generating a revised conditionvalue representing a time change in the running condition of the drivingwheels by decrementing a current wheel condition value by a first step(Δx_ΔB) each time a locking of the driving wheels is detected in thedetecting step, decrementing the current wheel condition value by asecond step (Δx_ΔE) each time a spinning of the driving wheels isdetected in the detecting step, and incrementing the current wheelcondition value by a third step (Δx_out) each time neither a locking ofthe driving wheels nor a spinning of the driving wheels is detected inthe detecting step, ii) comparing the revised condition value with areference condition value; and iii) based on the comparison of therevised condition value with the reference condition value, decidingwhether or not the instantaneous running condition corresponds to aslippery ground condition.
 15. A motor vehicle, comprising: an automaticgearbox with discrete gear ratios; a detector for detecting aninstantaneous running condition of driving wheels; and a processor fori) generating a revised condition value representing a time change inthe running condition of the driving wheels by decrementing a currentwheel condition value by a first step (Δx_ΔB) each time a locking of thedriving wheels is detected in the detecting step, decrementing thecurrent wheel condition value by a second step (Δx_ΔE) each time aspinning of the driving wheels is detected in the detecting step, andincrementing the current wheel condition value by a third step (Δx_out)each time neither a locking of the driving wheels nor a spinning of thedriving wheels is detected in the detecting step, ii) comparing therevised condition value with a reference condition value; and iii) basedon the comparison of the revised condition value with the referencecondition value, deciding whether or not the instantaneous runningcondition corresponds to a slippery ground condition.